The present invention relates to filters that are required for signal processing. The present invention relates, in particular, to an adaptive filter that is suitable for analog signal processing. Furthermore, the present invention relates to a low-pass filter.
An output signal on which the smallest possible noise voltage is superposed is demanded of analog sensors, for example analog magnetic field sensors. Furthermore, the output signal should follow rapid changes in the sensor input quantity with the shortest possible dead time. Unfortunately, these two requirements conflict with one another.
The requirement for a small noise power can generally be realized only by band limiting. In many analog sensors, this involves low-pass filtering since most of the physical quantities (temperature, magnetic field, pressure, acceleration, etc.) are of practical interest only for frequencies from 0 Hz (=temporally constant physical measurement quantity) up to a maximum frequency fmax, which generally lies between 100 Hz and 100 kHz. In this case, the low-pass filter is designed to transfer all of the spectral components in the useful frequency band from 0 Hz to fmax, as much as possible without distortion, from the input to the output. By contrast, all of the frequencies above fmax are attenuated to the greatest possible extent.
In this case, using the transfer function H(s)=Ua(s)/Ue(s) (s="sgr"+jxcfx89 where xcfx89=2xcfx80f; f . . . frequency; j . . . imaginary unit; Ua, Ue . . . Laplace transform of the output signal/input signal of the low-pass filter), undistorted transfer is understood to be obtained, depending on the application, when the amplitude of the transfer function is constant as much as possible, or when the phase shift xcfx86=arg (H(jxcfx89)) of the output signal with respect to the input signal is constant, as much as possible, and is as small as possible, or when the input signal has the smallest possible group delay Tgr=dxcfx86/dxcfx89 though the low-pass filter. Accordingly, different filter types, such as for example, Butterworth, Chebyshev, Bessel, etc, result in different optimization criteria result being taken into account for the transfer function of the filter.
What is common to all of the filters, however, is that the signal is delayed to a greater extent, the greater the narrowband nature of the low-pass filter. Thus, if the useful signal band is embodied such that it has a highly narrowband nature, then although this minimizes the noise power in the output signal, the dead time nonetheless rises simultaneously. The dead time is that temporal offset with which the sensor output reacts to a rapid change in the physical measurement quantity.
In particular in the case of integrated circuits which are produced in large numbers, a filter which is automatically adapted to the respective application is desirable. If the input quantity only changes very slowly, then the filter should have a highly narrowband nature and thereby minimize the noise power in the output signal. If the input quantity changes to a great extent and/or rapidly, however, then the low-pass filter should step up its cut-off frequency accordingly, so that the reaction time of the output signal is minimized.
For adaptive filters of this type, what are particularly unpleasant are sinusoidal input signals which change relatively rapidly at the zero crossing, but have practically no change during the reversal times. At these reversal times, the filter must not change its cut-off frequency because that would mean a considerable distortion (harmonic distortion factor) of the output signal. For the filter this means that, in the event of a slowing down of the input signal, it must not react immediately with a reduction of the cut-off frequency, but rather must observe the input signal over a longer period of time. Only if the input signal remains unchanged for a relatively long time is the cut-off frequency of the filter permitted to be reduced.
By way of example, if a 50 Hz signal is to be transferred without distortion, then this observation time period must at least be longer than the shallow crests of the 50 Hz signal (that is approximately 1/50/4 seconds=5 ms). However, such large time constants are virtually impossible to realize economically using integrated technology since they require large capacitances, and consequently, a great deal of chip area. Therefore, large filter time constants with a short dead time are generally realized by adaptive digital filters. However, if an analog signal has to be digitized specially for this, then this outlay is worthwhile only in a small number of cases.
It is accordingly an object of the invention to provide an adaptive filter and a low-pass filter which overcome the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention to provide an adaptive filter with a large time constant that requires only a small area and which can be used without digitizing the input signal. Furthermore, it is an object of the invention to provide a low-pass filter that likewise requires only a small area.
With the foregoing and other objects in view there is provided, in accordance with the invention, an adaptive filter for analog filtering a signal. The adaptive filter includes: a signal input; a signal output; and at least one potentiometer having a first terminal, a second terminal, and a third terminal. The second terminal is for subdividing the potentiometer into two series-connected resistors. The first terminal is connected to the signal input, and the second terminal is connected to the signal output. The adaptive filter also includes: at least one capacitor connected in series with the potentiometer via the third terminal of the potentiometer; and at least one control circuit for controlling the subdividing of the potentiometer into the two series-connected resistors.
The adaptive filter provides the advantage that it can be realized with a very low amount of circuitry, as a result of which, the adaptive filter becomes practical for the first time for many applications. In particular, the area required by the filter is small since the large time constants that are necessary for many applications can be realized in a simple manner by the control circuit. The adaptive filter is suitable in particular for xe2x80x9cmixed signal ASICSxe2x80x9d, in which the requirements for the control circuit have already been specified, so that the control circuit can be realized just by means of corresponding wiring.
The adaptive filter furthermore has the advantage that it can be used without digitizing the input signal. Moreover, the passive realization of the adaptive low-pass filter precludes an unavoidable offset error as caused by conventional active filters.
The adaptive filter includes a potentiometer having a second terminal which can be used to subdivide the potentiometer into two series-connected resistors. In this case, the limiting cases can also arise where the second terminal corresponds either to the first terminal or to the third terminal. However, the potentiometer of the adaptive filter can also be designed in such a way that the second terminal cannot be set exactly to the first and/or third terminal, so that the filter always has a finite bandwidth. Moreover, it may be advantageous, under certain circumstances, that even in the case of a narrowband signal in the signal path, a higher cut-off frequency can nevertheless be set than would be possible if the second terminal were set to the third terminal.
The potentiometer of the adaptive filter can also be designed in such a way that only two positions can be set for the second terminal, one position corresponding to a low cut-off frequency and the other position corresponding to a higher cut-off frequency. A changeover is then made between these two cut-off frequencies in accordance with the control circuit.
In accordance with an added feature of the invention, the control circuit has an evaluation circuit that forms the magnitude of the difference between the voltage at the first terminal of the potentiometer and the voltage at the third terminal of the potentiometer and compares it with a threshold voltage. This difference is a good measure of the changes in is input quantity.
In accordance with an additional feature of the invention, the control circuit has a counter, in particular an up/down counter. In this case, it is preferred if the counter can be reset to an initial value by a signal. In this case, it is particularly preferred if the counter is controlled by a clock signal and the counter can be reset to an initial value by a signal, asynchronously with the clock signal. Furthermore, it is preferred if an assessment circuit is provided, which monitors the resetting of the counter.
In accordance with another feature of the invention, the potentiometer includes a multiplicity of series-connected resistors. Furthermore, switches are provided, which can be used to shift the second terminal of the potentiometer between the resistors. This results in a simple realization of a potentiometer which can be driven by the counter via a decoder.
In accordance with a further feature of the invention, a feedback circuit is provided, which is connected to the capacitor to thereby form a filter of any desired order. In this case, it is particularly preferred if the feedback circuit contains a transfer element with a predetermined transfer function.
In accordance with a further added feature of the invention, at least two capacitors are provided, which are connected to the potentiometer. In this case, it is particularly preferred if a feedback circuit is provided, which is connected to at least one of the capacitors, thereby forming a filter of a predetermined order. Furthermore, it is preferred if the feedback circuit contains at least one transistor connected up as a source follower. The control terminal of the transistor is connected to the third terminal of the potentiometer. In this case, it is particularly preferred if the feedback circuit contains at least one transistor which has a gain of about 1 and/or which decouples the capacitor that is connected to the feedback circuit from the third terminal of the potentiometer.
With the foregoing and other objects in view there is provided, in accordance with the invention, a low-pass filter for analog filtering a signal. The low-pass filter includes: a signal input; a signal output; and an RC network including at least two resistors and at least two capacitors. The RC network provides a connection between the signal input and the signal output. The connection includes only the resistors. The low-pass filter also includes a feedback circuit connected to at least one of the capacitors to thereby define a predetermined order of the filter. The feedback circuit has a transistor connected as a source follower, and the transistor has a control terminal connected to the signal output.
The low-pass filter can be realized with a low circuitry outlay. In particular, the area required by the low-pass filter is small.
In accordance with a further feature of the invention, the transistor of the feedback circuit has a gain of about 1. Furthermore, it is preferred if the transistor decouples the capacitor that is connected to the feedback circuit from the signal output.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a filter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.